training vines on a tree

Happy Monday everyone!
I am looking for advice on training basic Clematis Paniculata to grow on an oak tree. This may be a silly question--but do I allow the vine to grow straight up, or encourage the vine to wrap around the tree? So far I have tied twine around the tree trunk to hold the vine in place, yet the vine does not seem to want to cling to the tree trunk.
Thanks! Heidi 7b
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